Method for the catalytic conversion of alkylene carbonate

ABSTRACT

A method for catalytic conversion of alkylene carbonate, wherein alkylene carbonate is contacted with C 1 -C 5  aliphatic alcohol and/or water in the presence of Mg, Al mixed (hydr) oxide catalyst having a Mg:Al molar ratio in the range of from 4 to 20.

This application claims the benefit of Application No. 01309560.9 filedon Nov. 13, 2001 in Europe under 35 U.S.C. § 119, § 365(a), or § 365(b).

1. Field of Invention

The present invention relates to a method for catalytic conversion ofalkylene carbonate, using an Mg, Al mixed (hydr) oxide catalyst, and acatalyst therefore.

2. Background of the Invention

EP-A-0 51 351 disclose an Mg, Al mixed (hydr) oxide catalyst having anMg: Al molar ratio above 3 and preferably in the range from 3-10.

The article of H. Schaper et al. in Applied Catalysis, 54, (1989) 79-90,discloses the same catalyst. This catalyst has a hydrotalcite structure,consisting of brucite type layers in which part of the bivalent ions(Mg) are replaced by trivalent ions, alternated by interlayers whichcontain water and anions to compensate for the excess charge of thetrivalent ions. The preparation of such catalysts is disclosed. Due tothe basic properties such catalysts are considered of special interestfor base-catalyzed reactions, such as polymerization of propylene oxide,double-bond isomerisations of olefins such as 1-pentene, and aldolcondensations. Exemplified is double-bond isomerisation of 1-penteneusing an Mg, Al mixed oxide catalyst having an Mg:Al molar ratio of 5and 10. At increasing molar ratio the conversion rate decreases.

The article of Watanabe, Y. et al. in Microporous and MesoporousMaterials 22 (1998) 399-407, discloses the use of Mg—Al hydrotalcitecatalysts having a molar ratio of 1.8-2.5 for the methanolysis ofethylene carbonate for the production of dimethyl carbonate.

EP-A-0,478,073 describes a process for preparing a dialkyl carbonatewhich comprises contacting an alkylene carbonate with an alkanol in thepresence of a mixed metal oxide catalyst or a modified bimetallic orpolymetallic catalyst under conditions effective to produce the dialkylcarbonate. In the examples, a magnesium/aluminium mixed metal oxidecatalyst having a Mg:Al ratio of 3:1 was employed.

In JP-A-06/238165, a process is described wherein an alkylene carbonateand an alcohol are subjected to transesterification in presence of acatalyst to produce a dialkyl carbonate. A combination of Magnesiumoxide and another metal oxide other than magnesium was used as catalystin an atomic ratio in the range of 1000:1 to 20:1 of magnesium to theother metal.

The present invention has for its object to provide a method for thecatalytic conversion of alkylene carbonate having an improved conversionrate and improved yield, while having limited leaching of metal from thecatalyst.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a method for catalytic conversion ofalkylene carbonate, wherein alkylene carbonate is contacted with C₁-C₅aliphatic alcohol and/or water in the presence of Mg, Al mixed oxidecatalyst having an Mg:Al molar ratio in the range of from 4 to 20.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention is based on the insight that by increasing the Mg:Al molarratio in this type of (hydrotalcite) catalyst conversion rate and yieldboth improve. Although in the preparation of the catalyst a so calledmixed Mg/Al hydroxide is formed it might be that under workingconditions mixed Mg/Al oxides also or only are present. The catalystwill be referred to as Mg, Al (hydr)oxide catalyst. At molar ratiosabove about 4, such as of from about 4 to about 20 Mg:Al and,preferably, above about 5 such as about 5 to about 20 the catalystexhibits the highest activity.

During the catalytic conversion of alkylene carbonate metal oxide, inparticular Mg, may leach from the catalyst particles. The leach rate isreduced when the Mg:Al molar ratio is below about 20, such as of fromabout 4 to about 20, preferably below about 10, such as of from about 4to about 10. The MG,Al (hydr) oxide catalyst suitable for the purpose ofthe present invention will therefore have a Mg:Al ratio of at leastabout 4, preferably more than about 4, even more preferably at leastabout 5, and most preferably more than about 5. The Mg,Al (hydr) oxidecatalyst will further preferably have a Mg:Al ratio of at most about 20,more preferably of less than about 20, even more preferably of at mostabout 15, again more preferably of less than about 15, most preferablyof at most about 10.

Alkylene carbonate suitable for use in the catalytic conversion methodaccording to the invention may be a C₁-C₅ alkylene carbonate such as 1,2and 1,3 propylene carbonate, 1,2 and 2,3 butadiene carbonate. Preferredare ethylene carbonate and propylene carbonate. C₁-C₅ aliphatic alcoholsuitable for use comprises a straight and branched C₁-C₅ alkanols.Preferred are methanol and ethanol. Most preferred is methanol. In thepresence of one or a mixture of C₁-C₅ alkanol, the alcoholysis resultsin the formation of di(C₁-C₅) alkyl carbonate and alkylene diol. In thepresence of only methanol the methanolysis results in the formation ofdimethyl carbonate and the alkylene diol. The catalytic conversion inthe presence of only water results by hydrolysis in the production ofthe alkylene diol and carbon dioxide. The catalytic conversion in thepresence of C₁-C₅ aliphatic alcohol and water results in the formationof all end products in ratio's dependent on the methanol:water molarratio. When the molar ratio of methanol:water is higher than about 6,such as from about 10 to about 20. The dimethyl carbonate:alkylene diolmolar ratio is between about 1 and about 0.

Another aspect of the invention relates to the use of this catalyst asdefined above for the catalyst conversion of alkylene carbonate withC₁-C₅ aliphatic alcohol and/or water, and in particular to themethanolysis and/or hydrolysis of alkylene carbonate.

The Mg, Al mixed (hydr) oxide catalyst has generally a Mg:Al molar ratioin the range of of from about 4 to about 20. At higher molar ratio'sleaching of the metal oxide, in particular Mg increases. For an optimalcatalyst activity at low Mg leaching the Mg:Al molar ratio is in therange of from about 4 to about 20, in particular of from about 5 toabout 10 or of from about 10 to about 20.

The method and use of the catalyst according to the invention will befurther elucidated by reference to the following examples which areprovided for illustrative purposes and to which the invention is notlimited.

EXAMPLE 1 Catalyst Preparation

The Mg/Al samples were prepared by semi-batch co-precipitation atconstant pH. Aqueous solutions of Mg(NO₃)₂.6H₂O (1 M) andAl₂(SO₄)₃.18H₂O (0.5 M) were prepared from demineralised water and mixedproportionally to the targeted Mg/Al molar ratio. The resulting solutionwas then added drop-wise to 600 ml of an aqueous solution of 25% NH₃(pH=9) under constant stirring at 65° C. The precipitating solution waskept at pH=9 by addition of 25% NH₃ solution. The slurry was then agedfor 1 hour under continuous stirring and filtered. The resulting pastewas washed with demineralised-water until the pH of the wash waterbecame neutral and, finally, dried over night at 80° C.

EXAMPLE 2 Catalyst Screening

The samples were evaluated in a unit equipped with 6 quartz reactorshaving an inner diameter of 3 mm. Catalyst charges of 0.15 gram (30-80mesh size) were diluted with 0.45 gram of SiC (0.05 mm diameter) andloaded into the reactor, with a pre-bed of 0.45 gram SiC placed on top.The catalysts were dried in situ under N_(2—) flow at 120° C. for 1hour. The reactors were then pressurised to 25-30 bar and the feed flowis started with a space velocity of WHSV=5 gr/(gr.hr), together with amoderate N₂ flow of WHSV=2 gr/(gr.hr). The liquid feed consisted of aPC:MeOH mixture of 1:4 molar ratio. After a stabilising period of 20hours at 120° C., the liquid products were condensed at 15° C. and 1 barduring a 24 hours run for off-line GC analysis. The moderate N₂ flowneeded for pressure regulation and product transport stripped some ofthe light ends from the sample. Therefore mass balances were only madeon propylene carbonate.

In the following examples, the conversion of methanol and yield ofdimethyl carbonate (DMC) are based on the molar amounts of thesecompounds divided by the molar amount of methanol supplied times 100%.The conversion of propylene carbonate to monopropyleneglycol (MPG)and/or methylpropanyl carbonate (MPC in mol %) and the yield of MPGand/or MPC are based on the molar amount of recovered PC divided by themolar amount of PC supplied in the feed times 100%.

TABLE 1 performance of Mg/Al (hydr) oxides for methanolysis of propenecarbonate (120° C., total liquid WHSV = 5 g/g/h, a N₂ flow of WHSV of 2g/g/h and 25 bar, catalyst calcined at 400° C. unless specifiedotherwise) Conv. Yield light Catalyst MeOH^(a) PC^(b) DMC^(a) MPG^(b)ends^(b) MPC^(b)  1 Mg/Al^(c) 8.9 12.4 1.7 6.4 0.3 5.9  2 Mg/Al 9.0 11.51.5 5.5 0.4 6.0  5 Mg/Al 15.1 21.1 3.8 15.4 0.3 5.7 10 Mg/Al 17.4 25.34.7 20.2 0.4 5.1 20 Mg/Al 26.0 35.8 8.3 34.1 0.1 1.6 50 Mg/Al 25.0 34.18.0 32.5 0.0 1.6 Mg (OH)₂ 6.6 8.6 1.8 7.0 0.0 1.6 Effect of calcinations 5 Mg/Al 15.1 21.1 3.8 15.4 0.3 5.7 calc. 400° C.  5 Mg/Al 16.6 23.44.35 18.2 0.4 5.1 calc. 80° C. 10 Mg/Al 17.4 25.3 4.7 20.2 0.4 5.1 calc.400° C. 10 Mg/Al 15.7 21.0 4.0 15.9 0.3 5.1 calc. 80° C. ^(a)dimethylcarbonate expressed in mole % based on methanol supplied in feed;^(b)monopropylene glycol and methyl propanyl carbonate (MPC) expressedin mole % based on propylene carbonate (PC) supplied in feed; ^(c)xMG/Alimplies a Mg:Al molar ratio of x.

According to table 1, the activity of the mixed Mg/Al (hydr) oxidesincreases with increasing Mg/Al ratio, exception made for the pureMg(OH)₂ which shows one of the lowest activity, possibly because ofleaching indicated by the formation of a hazy liquid product.

Calcining the materials at 80 or 400° C. prior to loading into thereactor had little influence on their catalytic performance. Thisillustrated for 5 Mg/Al and 10 Mg/Al in Table 1.

The 20 Mg/Al and 50 Mg/Al catalysts exhibit the highest activity, butdegrade to some extent during the reaction such that the catalyst/SiCbed was very hard to remove from the reactor. By contrast, the othersamples came out as free flowing particles. The Mg(OH)₂ sample was alsofree-flowing, though the haziness of the liquid product suggestssignificant leaching during the reaction.

Similar results have been obtained when using ethylene carbonate (EC)instead of propylene carbonate. Under the same operating conditions asapplied for the examples of table 1, except for the ethylene carbonatewhich now substitutes the propylene carbonate in the feed, the 5 Mg/Alcatalyst converted EC to EG with 28 mole % yield based on EC supplied infeed and a DMC/EG molar ratio of 0.89.

EXAMPLE 3 Catalyst Stability/Activity

In order to assess the stability of the various materials, samples of0.1 g of each catalyst were immersed in 15 ml of a representativeMeOH:PC:MPG mixture (3.46:0.88:0.24 molar ratio) for 20 hours at roomtemperature. Then 5 ml samples were taken from the top of the liquid andanalysed with ICP-spectrometry. The magnesium content of these productsincreased with the Mg/Al molar ratio starting at Mg/Al of ˜10 (Table 2).It is concluded that the 5 Mg/Al and 10 Mg/Al catalysts offer the bestcompromise between activity and stability in the reaction medium.

TABLE 2 Magnesium content of a MeOH:PC:MPG mixture (3.46:0.88:0.24 molarratio) after 20 hours immersion of various Mg/Al mixed hydroxide at roomtemperature. Mg leaching Catalyst [mg/kg] 1 Mg/Al 9.30 2 Mg/Al 3.52 5Mg/Al 10.14 10 Mg/Al 25.0 20 Mg/Al 34.5 50 Mg/Al 72.9 Mg 164.4

EXAMPLE 4 Performance in Methanolysis/Hydrolysis

The catalytic tests were carried out in a single-tube microflow unitwhich is equipped with a HPLC pump to feed the PC-MeOH-water mixture, agas manifold to introduce N₂ at 0.7 Nl/h, a traced feed line, astainless steel reactor of 15 mm ID (with thermowell) operating in downflow, a high-pressure condenser operating at room temperature and anautomatic sampling manifold that distributes the liquid productsequentially over six bottle of 300 ml.

The reactor was typically charged with 2 g of catalyst (1.6 mmcylinders) diluted in 15 g of SiC. Once loaded, the reactor was heatedup to reaction temperature (120-140° C.) under a N₂ flow of 0.7 Nl/h(i.e. WHSV of ˜0.4 g N₂/g cat/h) at 25-30 bar for 16 h. The reactor wasthen set to reaction temperature and pressure (25 bar), contacted withthe partially vaporised feed at target velocity (typically 2 g liq./gcat/h) and operated under varying conditions for more than 1000 hourswithout interruption.

The liquid product was analysed off-line by means of GC using the polarcolumn. The gas stream was not analysed. However, occasional use of acold trap (−60° C.) in the gas line did not provide more than 0.05 C %(based on total feed) of additional product, which appeared to be mainlymethanol upon immediate GC analysis.

TABLE 3 performance of 5 Mg/Al (hydr) oxide for methanolysis and/orhydrolysis of propene carbonate (140° C., total liquid WHSV = 2 g/g/h, aN2 flow of WHSV of 0.4 g/g/h and 25 bar) Feed MeOH: Conv. Yield lightDMC: H₂O:PC MeOH^(a) PC^(b) DMC^(a) MPG^(b) ends^(b) MPC^(b) MPG^(c)4:0:1 22.6 39.7 8.7 34.1 0.7 2.3 1.0 1:0:1 22.8 11.6 10.6 10.0 0.3 1.41.1 0.5:0:1 28.1 6.3 12.6 5.4 0.3 0.6 1.1 4:0:1 22.6 39.7 8.2 34.1 0.72.3 1.0 3.8:0.2:1 7.4 36.3 3.5 35.0 0.1 1.3 0.4 3.4:0.6:1 1.4 52.2 0.550.8 0.0 1.4 0.0 0.5:0:1 28.1 6.3 12.6 5.4 0.3 0.6 1.1 0.3:0.2:1 2.421.1 0.5 20.6 0.0 0.4 0.0 0:0.36:1 — 35.1 — 35.1 0.0 0.0 0.0^(a)expressed in mole % based on methanol supplied in feed;^(b)expressed in mole % based on PC supplied in feed; ^(c)expressed inmole:mole.

Table 3 shows that the 5 Mg/Al catalyst converts PC to MPG in thepresence of methanol and/or water in varying ratio. The catalyst wasstable for more than 1000 h and operated satisfactorily over a widetemperature range, from 120 to 180° C. and a variety of residence time,from 4 to 40 min (1/WHSV). By varying the feed composition it producedDMC/MPG in a ratio that varied from nearly 1:1 with a water-free feed to0:1 with a methanol-free feed. The hydrolysis reaction turns out toproceed at higher rate than the methanolysis reaction (compare e.g. thefeed of 0:0.36:1 with 0.5:0:1, respectively). In contrast to thehydrolysis reaction, the methanolysis is limited by thermodynamicequilibrium. These two phenomena result in a DMC:MPG molar ratio in theproduct that drops much faster than do the MeOH:H₂O molar ratio in thefeed. In other words, it suffices to substitute as small fraction ofwater for MeOH to uncouple the production of DMC from that of MPG.

The spent catalyst did not significantly differ from the fresh catalystas characterised by XRD, XPS and bulk element analysis. The absence ofsignificant chemical changes of 5 Mg/Al catalyst during the reaction ofPC with MeOH is consistent with the low Mg and Al content (<10 ppm) ofthe liquid product measured by means of ICP analysis. When normalised tothe production rate of MPG, the Mg leaching rate generally remainedbelow 100 ppm (i.e. <100 mg Mg/kg MPG) and more typically <50 ppm. Thislow leaching rate holds for MeOH:PC as well as H₂O;PC and MeOH:H₂O:PCfeeds.

Similar results are obtainable with other alkylene carbonates and C₂-C₅aliphatic alcohols.

1. A process for producing di(C₁-C₅) alkyl carbonate and alkylene diol,which process comprises the steps of contacting alkylene carbonate withC₁-C₅ aliphatic alcohol in the presence of a Mg, Al mixed (hydr) oxidecatalyst having a Mg:Al molar ratio in the range of from about 4 toabout
 20. 2. A process for producing alkylene diol and carbon dioxide,which process comprises the steps of contacting alkylene carbonate withwater in the presence of a Mg, Al mixed (hydr) oxide catalyst having aMg:Al molar ratio in the range of from about 4 to about
 20. 3. A productstream comprising di(C₁-C₅) alkylcarbonate and alkylene diol made by aprocess comprising the steps of contacting alkylene carbonate with C₁-C₅aliphatic alcohol in the presence of Mg, Al mixed (hydr) oxide catalysthaving a Mg:Al molar ratio in the range of from about 4 to about
 20. 4.A product stream comprising alkylene diol and carbon dioxide made by aprocess comprising the steps of contacting alkylene carbonate with waterin the presence of a Mg, Al mixed (hydr) oxide catalyst having a Mg:Almolar ratio in the range of from about 4 to about 20.